Porch fireplace chase going thru conditioned attic space
New build, zone 2. Plans call for a prefab’ed fireplace on the back screened porch, i.e., outside the building envelope. To be clear, the FP opening will be accessible from the porch only. The chimney chase will go through the house’s attic. The attic above the FP is unvented, part of the building envelope, and will probably have OC spray foam insulation on the underside of the roof sheathing. The FP “enclosure” below the attic will not be insulated since it is outside the building envelope.
Typically in this area (where attics in 99% of houses aren’t in the building envelope and insulation is blown in on the attic floor instead of the underside of the roof), the FP enclosure stops at the ceiling, the flue pipe is exposed in the attic, and an enclosed, uninsulated chase is built from the roof up for the outside flue pipe.
In order to keep the attic relatively tight and insulated as part of the building envelope, would it suffice to frame and sheath a flue chase from the attic floor to the roof, then either spray foam the outside of the sheathing or attach rockwool batts to the outside of the chase within the attic?
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